Everything to know about the coolant reservoir

You're cruising down the highway, wind in your hair, tunes blasting, when suddenly, your engine starts heating up like a sauna on wheels—your car's coolant system has a problem! As the unsung hero of your car's cooling system, the coolant reservoir may look like just an unassuming plastic tank, but it plays a key part in keeping your car’s engine running smoothly. It can also be an important indicator of potential issues with your ride.

So, buckle up as we dive into everything you need to know about coolant reservoirs, from bubbling, overflowing, and occasional oil-slicked surfaces, to what to do when your coolant reservoir is empty, and how to keep them in tip top condition.

What does a coolant reservoir do?

A seemingly humble container, the coolant reservoir (also known as an overflow tank) stores coolant, that magical elixir tasked with keeping your engine from melting into a puddle of regret. As your engine heats up, it makes your coolant expand, and it flows into the reservoir for safekeeping. Once you stop driving and your car begins to cool down, the coolant is sucked back into the radiator to start the cycle anew next time you fire up your vehicle.

Common issues with the coolant reservoirblown

Coolant reservoir bubbling

Bubble bubble—does it mean toil and trouble? Bubbles in the coolant reservoir can mean there’s a rise in air pressure in the cooling system, and whilst the most common culprit is a blown head gasket, there can be a number of other causes for coolant bubbling in the reservoir but not overheating that aren’t quite as dire. Other issues may be a faulty radiator cap or thermostat, damaged seals, or a sneaky leak in your coolant reservoir hose. Whatever the issue it is important to have it investigated promptly as early intervention can avoid potentially costly problems further down the road.

Can a bad thermostat cause bubbling in the coolant reservoir?

Yes, it can. The thermostat is responsible for controlling the flow of coolant in and out of the radiator in response to the engine temperature, and if this little contraption malfunctions it can wreak havoc on your engine's cooling system. When the thermostat fails to open and close at the right times it can lead to unregulated airflow and the dreaded bubble effect in your reservoir or radiator. This obviously spells trouble for your engine's temperature control, and could potentially lead to overheating and serious engine damage if left.

What if there’s oil in the coolant reservoir?

That rainbow slick sheen coating on top of your coolant may look pretty, but if you spy oil slicks in your reservoir, it means you have a serious issue that requires immediate attention. When a head gasket fails, oil can leak into the cooling passages and end up in the coolant, which is bad news for your engine.

Other engine issues that lead to oil in the coolant reservoir but your car not overheating could be a faulty oil cooler or damaged seals. Your first step is to identify where the oil is coming from and have that problem fixed, then have your car’s coolant system completely drained and flushed out to ensure no oil is left.

The final step is to have your car inspected for any damage and refill it with fresh coolant.

Why is my coolant reservoir overflowing?

From a damaged thermostat to water pump issues, there are several reasons why your reservoir might be overflowing. Firstly check the obvious and make sure you haven’t just overfilled the coolant. Next, double check the reservoir cap is screwed on tightly and not damaged, then do the same for the radiator cap. If the thermostat is malfunctioning or damaged, that will lead to improperly regulated fluid levels which can cause coolant reservoir overflow. Keep an eye out for signs of overflowing such as the outer walls of the reservoir being wet and/or puddles or drips of coolant appearing under the car.

Causes of coolant leaking from overflow reservoir

If you spot coolant leaking from the overflow reservoir, once again try to rule out the most simple causes first, like checking whether the reservoir tank has a crack or hole in it. Then check the reservoir and radiator caps are on correctly and aren’t damaged in any way. If you see coolant fluid on the ground and around the water pump, then the water pump has likely given up the ghost. Once you’ve worked out that the caps, thermostat and water pump aren’t the problem, then it’s likely that you have a problem with the radiator. If you ignore a faulty radiator, it’s likely to cause a lot of expensive damage from overheating, so it’s important to get it inspected as soon as possible.

How to empty a coolant reservoir

To empty the coolant reservoir you can syphon off the coolant using a syphon-hose, or by following the steps below to remove the tank:

Simple process of how to clean coolant reservoir

Without a little extra attention your coolant reservoir can become clogged up over time with dirt, mud and sludge that can impact on your coolant system, so it’s a good idea to give it a thorough clean out every now and then. Once you’ve removed and emptied the tank as above you can clean it as follows:

Can you put water in the coolant reservoir?

If your car overheats and the coolant level is low you can top up the reservoir (once your engine has cooled to a safe temperature of course!) to the maximum level indicator water, but this should only be attempted to get you to the closest mechanic as water does not cool the engine nearly as efficiently as coolant does. Besides that, water in your cooling system can cause corrosion and rust to build up over time, so it’s only recommended in an emergency.

Easy coolant reservoir tank replacement

If your tank is cracked, leaking or degraded then it’s best to undertake a coolant reservoir replacement. It’s a fairly simple DIY task—just follow the steps below:

Your tank replacement is now complete!

Still not sure? Drop into mycar for help

If you’re feeling a bit unsure and would like our friendly team to help, then head on down to your local mycar—not only are we the coolant experts, but with over 275 stores located around Australia, we’re always conveniently located right where you need us for professional servicing, testing and replacement. And you can rest easy knowing that all our work is covered by a lifetime workmanship guarantee, giving you peace of mind.

So give us a call on 13 13 28 for all your coolant reservoir needs today!